1. Field of the Invention
The present invention relates to an image forming apparatus having a plurality of developing devices, and more specifically relates to an image forming apparatus having movable developing devices.
2. Description of the Related Art
Image forming apparatus have been proposed provided with a developing unit having a plurality of developing devices wherein said developing unit is moved, then a specified developing device is stopped so as to be positioned at a predetermined developing position, and a developing operation is subsequently accomplished using said developing device.
In the image forming apparatus of the aforementioned type, the aforesaid developing unit must be moved at high speed and accurately stopped at a predetermined position because the time required to change the developing devices controls the copying speed.
Therefore, the image forming apparatus of the aforementioned type provides a control means for controlling the movement of a specified developing device to the developing position, and a unit driving means for moving and stopping the developing unit in accordance with movement control signals transmitted from said control means.
The mechanical characteristics of the aforesaid unit driving means (friction, resistance to moving the developing unit, and the like) are changed by changes in the weight of the developing unit due to consumption of the toner accommodated within the developing devices, environmental factors such as temperature and the like, or deterioration of a constant output spring used for balancing the device or the like. That is, deviations from the normal state may occur in the aforesaid mechanical characteristics. The normal state may be, for example, a state wherein a predetermined amount of toner is accommodated in each developing device, i.e., a state wherein the aforesaid spring or the like is set at a regulated value prior to shipping from the factory.
When deviations from the normal state occur as previously mentioned, accurately controlling the movement at high speed becomes difficult. A reason for the aforesaid difficulty is that the table data for regulating the movement control signals are set so as to be compatible with the normal state. Therefore, when the mechanical characteristics change and the deviations from the normal state occur, the actual moving speed of the developing unit is a value different from the speed achievable in the normal state and, accordingly, high speed and accurate movement control are difficult to achieve.
The aforementioned mechanical deviations are conventionally managed by methods such as:
(a) absorbing said deviations by using a large-capacity developing unit drive motor; PA1 (b) mounting a regulating mechanism on the unit driving means which is periodically adjusted service personnel as needed.
However, the effectiveness of the aforesaid methods is inadequate and labor intensive.